Headphones for nonsoftware monitoring

[dogen]

I am just now setting up a Digi001 with a G4. It's my first studio setup, so I've got a pretty basic question or two. I'd like to monitor a few types of things. When recording a track in ProTools LE, the headphone jack is usable. Plus I can play what I've recorded through the monitor outputs to my stereo. But say I want to play MP3s through the stereo or headphones by way of the Digi. I can push in the monitor button and listen through the stereo speakers, but the headphones don't play at all. So my question is, will the headphone jack on the front of the Digi only take a signal from ProTools or another app with an ASIO driver? If that's so, I imagine if I hook up my guitar synth outputs to the monitor ins (where I'd put the speaker output of the Mac to listen through the stereo), I won't hear anything. So is it then common practice to use a mixer before the Digi? Remember, I'm new to this, but I'm picturing the guitar synth, Digi, and computer speaker out all going into a mixer, then the mixer out going to the stereo amp and speakers. Or is a mixer an unnecessary additional component?

Phew! My first post. Hope it's followable.

Jon